1. Field of the Invention
The present invention relates to a method for depositing a multilayer film of a mask blank for EUV (Extreme Ultra Violet) lithography by using an ion beam sputtering method, and a method for producing a mask blank for EUV lithography.
2. Discussion of Background
A reflective coating of a mask blank for EUV lithography (hereinbelow, referred to as “EUV mask blank” in Description) is produced by depositing a multilayer reflective coating, wherein Mo layers and Si layers are alternately laminated on a superpolished substrate. In order to fabricate an EUV mask blank at a reasonable cost, such a multilayer reflective coating is required to satisfy the following characteristics:
Being free of a defect having a size of a certain value or above (a size of 32 nm or above in the case of, e.g., a mask used for fabricating semiconductor devices having a circuit line width of 32 nm) in a quality-assured range (142×142 mm square at a central portion in the case of a 6 inch mask).
Having a high reflectivity in a stable and uniform way in the quality-assured range with respect to the wavelength of a light source (normally 13.5 nm)
In a case wherein a fine irregularity exists on a substrate, when an attempt is made to print a desired pattern of a mask on a photosensitive organic film (a so-called photoresist film) on a Si wafer by using an exposing system, a portion of the desired pattern is defective, or an excessive pattern other than the desired pattern is formed in some cases since the periodic structure of a multilayer reflective coating formed on the substrate is perturbed. The perturbation in the periodic structure of a multilayer reflective coating, which is caused by an irregularity existing on a substrate, is called a phase defect, which causes a important problem. It is desired that no irregularity having a size of a certain value or above exist on a substrate. Non-patent documents 1 and 2 describe requirements relating to a defect in an EUV mask and an EUV mask blank. The requirements for the defect are significantly severe. Non-patent document 1 describes that a defect having a size of 50 nm or above is not permissible since the presence of such a defect on a substrate perturbs the structure of a reflective coating to form an unexpected shape in a pattern projected onto a resist on a Si wafer. Additionally, non-patent document 1 describes that the surface roughness of a substrate needs to be less than 0.15 nm in RMS (root-mean-square) in order to prevent the roughness of a line edge from increasing in a pattern projected onto a resist on a Si wafer.
Non-patent document 2 describes that it is not permissible that a defect having a size of 25 nm or above exists on a reticle having a reflective coating, which is used in EUV lithography.
Non-patent document 3 describes what size of defect on a substrate can be printed. Non-patent document 3 describes that there is a possibility that a phase defect modifies a line width of a printed image. A phase defect including a surface bump having a height of 2 nm and a FWHM (full width of half maximum) of 60 nm is a borderline phase defect as to whether the phase defect can be printed or not. This document describes that a defect having such a size causes an impermissible change of 20% in line width (140 nm on a mask) with respect to a line having a width of 35 nm.
Non-patent document 4 describes defect repair for EUVL mask blanks. Non-patent document 4 describes that a defect caused in the vicinity of the bottom of a reflective coating tends to change the phase of reflected light, and that such a defect is called a phase defect. The perturbation in the structure of a reflective coating caused by a phase defect can be reduced by irradiation of an electron beam having a high resolution. An increase in the temperature at an irradiated spot causes a silanization reaction between Mo and Si, leading to volume compaction.
Several techniques have been proposed for satisfying the severe requirements for such a defect stated above. One of the techniques is detect smoothing by use of a secondary ion source, and another one is use of an underlayer. Non-patent document 5 describes the former one, and the patent documents 1 and 2 describe the latter one.
Non-patent document 5 describes a method for smoothing out a thin film by an ion beam given by a secondary ion source, which has been developed for mitigating the effect of minute contaminants on a substrate, and which is called “defect smoothing”. In this method, a sputtering method, which is assisted by a primary ion source, is utilized to additionally deposit a Si film having a thickness of 1.4 nm or below, and a sputtering method using a secondary ion source is utilized to remove the deposited Si film by etching. By utilizing this method, foreign particles having a particle size of about 50 nm on a substrate can be smoothed out so as to have a height of 1 nm or below without having an adverse effect on the substrate.
Patent document 1 describes that an underlayer comprising a multilayer film is deposited as a film between a substrate and a reflective coating in order to make a defect size on a substrate as small as possible. This document also describes that the reduction in the defect size can be controlled by surface relaxation during the underlayer growth process using an ion beam sputtering system and by volume contraction of the materials at the multilayer interfaces.
Patent document 2 describes that an underlayer is disposed between a substrate and a reflective coating in order to make a defect size on a substrate as small as possible. The underlayer is disposed by a plurality of sequentially deposited and annealed coatings on the substrate. The annealing process can cause volume contraction of the coating material, mitigating the effect caused by a substrate defect.
Non-patent document 1: SEMI, P37-1102 (2002), “Specification for extreme ultraviolet lithography mask substrate”
Non-patent document 2: SEMI, P38-1102 (2002), “Specification for absorbing film stacks and multilayers on extreme ultraviolet lithography mask blanks”
Non-patent document 3: SPIE, vol. 4889, Alan Stivers., et. al., p. 408-417 (2002), “Evaluation of the Capability of a Multibeam Confocal Inspection System for Inspection of EUVL Mask Blanks”
Non-patent document 4: SPIE, vol. 5037, Stefan P. Hau Riege., et. al., p. 331-338 (2002), “Defect repair for extreme ultraviolet lithography (EUVL) mask blanks”
Non-patent document 5: Journal of Microlith., Microfab., and Microsys. 3, 139 (2004), Paul B. Mirkarimi, et. Al., “Developing a viable multilayer coating process for extreme ultraviolet lithography reticles”
Patent document 1: U.S. Pat. No. 6,319,635
Patent document 2: U.S. Pat. No. 6,489,066